Perth temperature history

It's worth digging a bit deeper into claims by the Bureau of Meteorology that Perth endured its equal hottest summer in 2010/11 and its hottest first four months of the year on record.

On May 2, 2011, The West Australian newspaper published the following story, claiming that "Perth has experienced its hottest start to the year on record, with a mean maximum temperature of 31.5C for the first four months, breaking the previous record of 31.3C set in 1978".

Oh dear, Tim Flannery was right and Perth really is at the forefront of global warming.

But can we trust the BoM calculations? Was the mean maximum in the first four months of 2011 really 31.5C?

Well, we can do it the BoM way and calculate the average of the rounded monthly figures for January (32.5C), February (34.1C), March (31.9C) and April (27.3C). Yep, that's an average 31.45C, which further rounds out to 31.5C. Too easy.

But what if we don't average the rounded monthly figures and instead average the actual maximum temperatures recorded on each day during the first four months? It's pretty easy to do. Just check the monthly figures for Perth Metro (station 9225), which is the official "Perth" site for the BoM, for January, February, March and April 2011.

The actual maximum temperatures in all 120 days of January, February, March and April 2011 can be seen here. Get your spreadsheets busy and figure out the average maximum. You'll find it's 31.42, which rounds to 31.4C.

So the actual mean maximum for Perth in the first four months of 2011 was 31.4C, not 31.5C as claimed by the BoM.

2010/11 was the third hottest summer recorded in Perth, but not the equal record.

Temperature station location

Ah, but this is being a bit pedantic, is it not, since the first four months of 2011 were still the hottest on record?

That depends upon whether you also want to research a bit deeper into the record itself. The BoM references its official "Perth" temperatures from station 9225 in Mt Lawley, which opened and recorded its first annual temperature means in 1994 - picture below in May 2011 courtesy Stephen Harper.

However, the BoM quotes the previous hottest first four months from 1978. So where was this location?

Before 2004, the official Perth temperatures were recorded at Perth Regional Office (station 9034). Check the first four months of 1978 and the mean maximum was 31.3C, the previous record as stated by the BoM.

Stations 9034 and 9225 are about four kilometres apart and the evidence suggests the Mt Lawley site is slightly hotter during the day than the Perth Regional Office site. Site 9034, which opened in 1876 and closed in 1992, has a 19 metre elevation.

Site 9225, which has had a stable mean temperature since opening in 1993, has an elevation of 25 metres. Proximity to broad river waters and the UHI influence of inner city and university development are probably relevant.

Below are the average annual maxima of station 9034 from 1975 until 1992, bleeding into station 9225 from 1994:

The average annual maximum at Perth Metro over 17 years is about .4C warmer than it was earlier at Perth Regional Office. If you suspect the increase is because of global warming, not the four kilometre change in location, consider the annual minima in "Perth" from 1975 to 2010:

The average annual minimum at 9225 over the past 17 years was about 1.6C cooler than it had been at 9034, the drop happening immediately the station change was made. Maybe a rapid onset of global cooling, but a more robust explanation is that the four kilometre change in station locations significantly affected temperature recordings and made historic pre-1994 comparisons dubious at best and irrelevant at worst.

The temperature record at Perth Regional Office

Perth Regional Office (9034) started recording from 1897 and finished in 1992. This location was originally known as the Observatory and started recording temperatures in 1897 at the Mt Eliza site now mostly occupied by Dumas House, adjacent to Kings Park. Although the BoM gives no clues that Perth Regional Office has a history of more than one location, station 9034 is the benchmark for historic temperature comparisons in the capital of Perth.

View of Observatory grounds soon after completion with Fraser Avenue and Kings Park in the background. The meteorological instruments including a Stevenson Screen can be seen in the right of the photograph.

Most of the Observatory was knocked down in 1967 to make way for Dumas House, and at that time the Commonwealth transferred its meteorological instruments to Wellington St in East Perth - about two kilometres away. (Source: Perth Observatory History)

The Bureau of Meteorology Perth Regional Office at 127 Wellington St, East Perth, facing north with the instrument enclosure in the foreground

The elevation changed from about 61 metres at the Observatory to 19 metres in Wellington St ...

... and look what happened to the temperatures recorded for Perth Regional Office (9034) ...

The trend is a bit more obvious just looking at the mean temp for Perth Regional Office ...

It should be noted that in 1963, Perth Regional Office was moved about 300 metres to the east of the Old Hale School building, also atop Mt Eliza (Source: Perth's Observing Sites)

And for Perth's temperature record from 1897 to 2010 ...

So Perth Regional Office changed its physical location in 1963 and again in 1967, when it moved about two kilometres east into the central city, closer to the river both in distance and elevation but separated by blocks of large buildings, and Perth's temperatures increased thereafter.

To further illustrate the distortion caused by moving the Stevenson screen temperature recording instrument a few kilometres from its original location, the Mt Lawley 9225 temperatures four kilometres inland show higher maxima and significantly lower minima.

Based on the temperatures charted above, the last two decades in Perth have had the hottest days and coldest nights since records began in 1897. Why are records claimed for average maxima but minima and mean are ignored, and why are any of them relevant for comparison with other locations before 1994?

It's worth looking a little more closely at Perth Metro ...

As demonstrated in the chart above, Perth's mean temperature has neither increased or decreased since 1994 when Perth Metro station 9225 was established in the suburb of Mt Lawley. In other words, Perth's "official" temperature hasn't changed since the first recording was made at a fixed location not subject to altitude, river proximity and inner city urban heat island deviations.

The 1912 Hunt document also contains a table with monthly and annual mean temperature readings at Perth Observatory from 1897 to 1907, and at Perth Gardens from 1876 to 1907, which provides a useful 11 year overlap.

The 11 year mean from 1897 to 1907 at Perth Observatory atop Mt Eliza is 63.9F (17.72C) and the same 11 year mean at Perth Gardens is 64.9F (18.27C). That's almost 0.6C difference in the same city in the same weather in the same years. The Perth Gardens did not record temperatures from a Stevenson Screen and this may have artificially increased the temperatures, but Commonwealth Meteorologist Henry Hunt suggests the thermometer was well shaded:

The mean temps for Perth Regional Office from 1897 to 1907 (go to BoM Climate Data and look up min/max for station 9034) are exactly the same as the mean temps for the Observatory in the 1912 Hunt document. These identical temperatures confirm that Perth Observatory on Mt Eliza was the original location for Perth Regional Office temps in its early years starting 1897.

The difference between station locations and the influence of inner city heat is further illustrated (Excel download 25kb) by comparing Perth Regional Office 9034 with Perth Airport 9021, about nine kilometres to the east.

Perth Airport opened in 1944 while Perth Regional Office relocated from Mt Eliza to inner Wellington St in 1967 and closed in 1992.

From 1944 to 1967, the average monthly minimum at Perth Airport was 12.1C and from 1967 to 1992 it was 12.1C - no difference. From 1944 to 1967, the average monthly maximum was 24C and from 1967 to 1992 it was 24.1C - a .1C warming.

From 1944 to 1967, the average monthly minimum at Perth Regional Office was 13.3C and from 1967 to 1992 it was 13.8C - a .5C warming. From 1944 to 1967, the average monthly maximum was 23.3C and from 1967 to 1992 it was 23.8C - a .5C warming.

Autumn 2011

On May 31 2011, the BoM stated in its seasonal update for Perth that autumn 2011 had the hottest mean maximum on record:

The BoM also references the average number of days above 30C at Perth Metro "from observations beginning in 1897". Perth Metro did not open until 1993. Perth Observatory atop Mt Eliza opened in 1897, again four kilometres away.

Conclusion

When informing the public that record high temperatures have been experienced, the Bureau of Meteorology should ensure the accuracy of its figures by averaging daily temperatures instead of rounded monthly calculations.

Rounded averages of rounded averages are not accurate and a .1C error is significant if claiming record temperatures, particularly when Australia's economic and political future is likely to be determined by a public understanding of the need or otherwise for a carbon dioxide tax.

The BoM should also acknowledge that temperatures recorded at its Mt Lawley 9225 station are being compared with stations in different locations where local environmental factors have a significant influence on Stevenson screen thermometers.

Perth's temperature history is too disjointed to make meaningful comparisons and present-day recordings at Mt Lawley should only be compared to recordings from the same site beginning in 1994.

It might mean Perth has a very short history of temperature recordings, but at least the data will be relevant.